Polish SpEL operator examples in reference docs

Closes gh-36367

Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

framework-docs/src/main/java/org/springframework/docs/core/expressions/languageref/expressionsoperatorsoverloaded/ListConcatenation.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2026-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.core.expressions.languageref.expressionsoperatorsoverloaded;
+
+import org.springframework.expression.Operation;
+import org.springframework.expression.OperatorOverloader;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ListConcatenation implements OperatorOverloader {
+
+	@Override
+	public boolean overridesOperation(Operation operation, Object left, Object right) {
+		return operation == Operation.ADD && left instanceof List && right instanceof List;
+	}
+
+	@Override
+	@SuppressWarnings({"rawtypes", "unchecked"})
+	public Object operate(Operation operation, Object left, Object right) {
+		if (operation == Operation.ADD &&
+				left instanceof List list1 && right instanceof List list2) {
+
+			List result = new ArrayList(list1);
+			result.addAll(list2);
+			return result;
+		}
+		throw new UnsupportedOperationException(
+				"No overload for operation %s and operands [%s] and [%s]"
+						.formatted(operation, left, right));
+	}
+}

framework-docs/src/main/kotlin/org/springframework/docs/core/expressions/languageref/expressionsoperatorsoverloaded/ListConcatenation.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2026-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.core.expressions.languageref.expressionsoperatorsoverloaded
+
+import org.springframework.expression.Operation
+import org.springframework.expression.OperatorOverloader
+
+class ListConcatenation: OperatorOverloader {
+
+	override fun overridesOperation(operation: Operation, left: Any?, right: Any?): Boolean {
+		return operation == Operation.ADD && left is List<*> && right is List<*>
+	}
+
+	override fun operate(operation: Operation, left: Any?, right: Any?): Any {
+		if (operation == Operation.ADD && left is List<*> && right is List<*>) {
+			return left + right
+		}
+
+		throw UnsupportedOperationException("No overload for operation $operation and operands [$left] and [$right]")
+	}
+}